Tissue PO2 Research Articles

The role of tissue oxygenation in obesity-related cardiometabolic complications

AbstractObesity is a complex, multifactorial, chronic disease that acts as a gateway to a range of other diseases. Evidence from recent studies suggests that changes in oxygen availability in the microenvironment of metabolic organs may exert an important role in the development of obesity-related cardiometabolic complications. In this review, we will first discuss results from observational and controlled laboratory studies that examined the relationship between reduced oxygen availability and obesity-related metabolic derangements. Next, the effects of alterations in oxygen partial pressure (pO2) in the adipose tissue, skeletal muscle and the liver microenvironment on physiological processes in these key metabolic organs will be addressed, and how this might relate to cardiometabolic complications. Since many obesity-related chronic diseases, including type 2 diabetes mellitus, cardiovascular diseases, chronic kidney disease, chronic obstructive pulmonary disease and obstructive sleep apnea, are characterized by changes in pO2 in the tissue microenvironment, a better understanding of the metabolic impact of altered tissue oxygenation can provide valuable insights into the complex interplay between environmental and biological factors involved in the pathophysiology of metabolic impairments. This may ultimately contribute to the development of novel strategies to prevent and treat obesity-related cardiometabolic diseases.

Consistent differences in tissue oxygen levels across 15 insect species reflect a balance between oxygen supply and demand and highlight a hitherto unknown adaptation for extracting sufficient oxygen from water

Animals, including insects, need oxygen for aerobic respiration and eventually asphyxiate without it. Aerobic respiration, however, produces reactive oxygen species (ROS), which contribute to dysfunction and aging. Animals appear to balance risks of asphyxiation and ROS by regulating internal oxygen relatively low and stable, but sufficient levels. How much do levels vary among species, and how does variation depend on environment and life history? We predicted that lower internal oxygen levels occur in insects with either limited access to environmental oxygen (i.e., insects dependent on aquatic respiration, where low internal levels facilitate diffusive oxygen uptake, and reduce asphyxiation risks) or consistently low metabolic rates (i.e., inactive insects, requiring limited internal oxygen stores). Alternatively, we predicted insects with long life-stage durations would have internal oxygen levels > 1 kPa (preventing high ROS levels that are believed to occur under tissue hypoxia). We tested these predictions by measuring partial pressures of oxygen (PO2) in tissues from juvenile and adult stages across 15 species comprising nine insect orders. Tissue PO2 varied greatly (from 0 to 18.8 kPa) and variation across species and life stages was significantly related to differences in habitat, activity level, and life stage duration. Individuals with aquatic respiration sustained remarkably low PO2 (mean = 0.88 kPa) across all species from Ephemeroptera (mayflies), Plecoptera (stoneflies), Trichoptera (caddisflies), and Diptera (true flies), possibly reflecting a widespread, but hitherto unknown, adaptation for extracting sufficient oxygen from water. For Odonata (dragonflies), aquatic juveniles had higher PO2 levels (mean = 6.12 kPa), but these were still lower compared to terrestrial adults (mean = 13.3 kPa). Follow-up tests in juvenile stoneflies showed that tissue PO2 remained low even when exposed to hyperoxia, suggesting that levels were down-regulated. This was further corroborated since levels could be modulated by ambient oxygen levels in dead individuals. In addition, tissue PO2 was positively related to activity levels of insect life stages across all species and was highest in stages with short durations. Combined, our results support the idea that internal PO2 is an evolutionarily labile trait that reflects the balance between oxygen supply and demand within the context of the environment and life-history of an insect.

Transient Changes in Cerebral Tissue Oxygen, Glucose, and Temperature by Microstrokes: A Computational Study.

This study focuses on evaluating the disruptions in key physiological parameters during microstroke events to assess their severity. A mathematical model was developed to simulate the changes in cerebral tissue pO2, glucose concentration, and temperature due to blood flow interruptions. The model considers variations in baseline cerebral blood flow (CBF), capillary density, and blood oxygen/glucose levels, as well as ambient temperature changes. Simulations indicate that complete blood flow obstruction still allows for limited glucose availability, supporting nonoxidative metabolism and potentially exacerbating lactate buildup and acidosis. Partial obstructions decrease tissue pO2, with minimal impact on glucose level, which can remain almost unchanged or even slightly increase. Reduced CBF, capillary density, or blood oxygen due to aging or disease enhances hypoxia risk at lower obstruction levels, with capillary density having a significant effect on stroke severity by influencing both pO2 and glucose levels. Conditions could lead to co-occurrence of hypoxia/hypoglycemia or hypoxia/hyperglycemia, each worsening outcomes. Temperature effects were minimal in deep brain regions but varied near the skull by 0.2-0.8°C depending on ambient temperature. The model provides insights into the conditions driving severe stroke outcomes based on estimated levels of hypoxia, hypoglycemia, hyperglycemia, and temperature changes.

Treatment of Hypotension with Phenylephrine Increases Cerebral Microvascular Perfusion in a Rat Model of Acute Hemodilution

Intraoperative blood loss is associated with reduced mean arterial blood pressure (MAP), which may endanger vital organ perfusion, including the brain. Phenylephrine (PE), an α1-receptor agonist, is clinically utilized to treat intraoperative hypotension through vasoconstriction of resistance arterioles. Clinical studies suggest that raising MAP with PE may decrease perfusion to vital organs such as the brain. Experimental studies suggest that lower α-receptor density in the cerebral resistance arteries may lead to preserved brain perfusion after PE administration. We hypothesize that treatment of hypotension with PE will result in preserved brain perfusion due to different microvascular blood flow and tissue oxygen tension (pO2) responses in the brain and skeletal muscle in an experimental rat model. With Animal Care and Use Committee approval in accordance with ARRIVE-2 guidelines, spontaneously breathing anesthetized rats (1.5% isoflurane) underwent hemodilution by exchanging 40-50% estimated blood volume 1:1 with saline over 10 minutes to achieve a 10-20 mmHg drop in MAP. MAP (tail artery) was measured continuously. Hypotension was treated with PE (10-20 μg/kg/min iv) to increase MAP to a target above baseline MAP. Shed blood was re-infused after stopping the PE infusion. Microvascular blood flow (Oxyflo; Oxford Optronix, Oxford, UK, n=7) or microvascular pO2 (OxyLED, Oxyphor G4, Oxygen Enterprises Ltd., Philadelphia, PA, n=6) were measured continuously in the cerebral cortex (burr hole) and biceps femoris muscle simultaneously in each rat. Results were analyzed via one or two-way ANOVA, with significance achieved at p<0.05. Baseline and post-hemodilution MAP values were comparable in both groups. PE infusion increased MAP to a mean level near 100 mmHg in both groups (p<0.05). Microvascular blood flow and tissue pO2 were greater in the brain as compared to the muscle at all time points (p<0.01 for both). PE infusion resulted in an increase in cerebral microvascular blood flow from baseline (1465±689 vs. 687±453 blood perfusion units (BPU); p<0.001) that was sustained after discontinuing PE and during blood reinfusion. No changes in muscle microvascular flow were observed after PE infusion (294±129 vs. 168±113 BPU). A sustained increase in cerebral microvascular pO2 was measured following PE infusion (50.2±8.6 vs. 40.6±4.6 mmHg; p=0.02), whereas muscle tissue pO2 did not change (22.2±5.6 vs. 25.0±2.5 mmHg). A differential blood flow and microvascular pO2 response was observed following PE infusion in brain and skeletal muscle. Both cerebral microvascular blood flow and tissue pO2 demonstrated sustained increases after PE infusion. By contrast, no changes were observed in muscle microvascular blood flow and pO2. These results suggest that treatment of hypotension with PE administration preferentially increased cerebral vs. muscle perfusion in an experimental model of acute blood loss and fluid resuscitation. Canadian Anesthesiologists’ Society and University of Toronto Merit Award Support. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

A novel approach to determine the critical survival threshold of cellular oxygen within spheroids via integrating live/dead cell imaging with oxygen modeling.

Defining the oxygen level that induces cell death within 3-D tissues is vital for understanding tissue hypoxia; however, obtaining accurate measurements has been technically challenging. In this study, we introduce a noninvasive, high-throughput methodology to quantify critical survival partial oxygen pressure (pO2) with high spatial resolution within spheroids by using a combination of controlled hypoxic conditions, semiautomated live/dead cell imaging, and computational oxygen modeling. The oxygen-permeable, micropyramid patterned culture plates created a precisely controlled oxygen condition around the individual spheroid. Live/dead cell imaging provided the geometric information of the live/dead boundary within spheroids. Finally, computational oxygen modeling calculated the pO2at the live/dead boundary within spheroids. As proof of concept, we determined the critical survival pO2 in two types of spheroids: isolated primary pancreatic islets and tumor-derived pseudoislets (2.43 ± 0.08 vs. 0.84 ± 0.04 mmHg), indicating higher hypoxia tolerance in pseudoislets due to their tumorigenic origin. We also applied this method for evaluating graft survival in cell transplantations for diabetes therapy, where hypoxia is a critical barrier to successful transplantation outcomes; thus, designing oxygenation strategies is required. Based on the elucidated critical survival pO2, 100% viability could be maintained in a typically sized primary islet under the tissue pO2 above 14.5 mmHg. This work presents a valuable tool that is potentially instrumental for fundamental hypoxia research. It offers insights into physiological responses to hypoxia among different cell types and may refine translational research in cell therapies.NEW & NOTEWORTHY Our study introduces an innovative combinatory approach for noninvasively determining the critical survival oxygen level of cells within small cell spheroids, which replicates a 3-D tissue environment, by seamlessly integrating three pivotal techniques: cell death induction under controlled oxygen conditions, semiautomated imaging that precisely identifies live/dead cells, and computational modeling of oxygen distribution. Notably, our method ensures high-throughput analysis applicable to various cell types, offering a versatile solution for researchers in diverse fields.

Differential responses of cerebral and renal oxygenation to altered perfusion conditions during experimental cardiopulmonary bypass in sheep.

We tested whether the brain and kidney respond differently to cardiopulmonary bypass (CPB) and to changes in perfusion conditions during CPB. Therefore, in ovine CPB, we assessed regional cerebral oxygen saturation (rSO2 ) by near-infrared spectroscopy and renal cortical and medullary tissue oxygen tension (PO2 ), and, in some protocols, brain tissue PO2 , by phosphorescence lifetime oximetry. During CPB, rSO2 correlated with mixed venous SO2 (r = 0.78) and brain tissue PO2 (r = 0.49) when arterial PO2 was varied. During the first 30 min of CPB, brain tissue PO2 , rSO2 and renal cortical tissue PO2 did not fall, but renal medullary tissue PO2 did. Nevertheless, compared with stable anaesthesia, during stable CPB, rSO2 (66.8 decreasing to 61.3%) and both renal cortical (90.8 decreasing to 43.5 mm Hg) and medullary (44.3 decreasing to 19.2 mm Hg) tissue PO2 were lower. Both rSO2 and renal PO2 increased when pump flow was increased from 60 to 100 mL kg-1 min-1 at a target arterial pressure of 70 mm Hg. They also both increased when pump flow and arterial pressure were increased simultaneously. Neither was significantly altered by partially pulsatile flow. The vasopressor, metaraminol, dose-dependently decreased rSO2 , but increased renal cortical and medullary PO2 . Increasing blood haemoglobin concentration increased rSO2 , but not renal PO2 . We conclude that both the brain and kidney are susceptible to hypoxia during CPB, which can be alleviated by increasing pump flow, even without increasing arterial pressure. However, increasing blood haemoglobin concentration increases brain, but not kidney oxygenation, whereas vasopressor support with metaraminol increases kidney, but not brain oxygenation.

EPR Monitoring of Oxygenation Levels in Tumors After Chlorophyllide-Based Photodynamic Therapy May Allow for Early Prediction of Treatment Outcome

PurposeMolecular oxygen, besides a photosensitizer and light of appropriate wavelength, is one of the three factors necessary for photodynamic therapy (PDT). In tumor tissue, PDT leads to the killing of tumor cells, destruction of endothelial cells and vasculature collapse, and the induction of strong immune responses. All these effects may influence the oxygenation levels, but it is the vasculature changes that have the main impact on pO2. The purpose of our study was to monitor changes in tumor oxygenation after PDT and explore its significance for predicting long-term treatment response.ProceduresElectron paramagnetic resonance (EPR) spectroscopy enables direct, quantitative, and sequential measurements of partial pressure of oxygen (pO2) in the same animal. The levels of chlorophyll derived photosensitizers in tumor tissue were determined by transdermal emission measurements.ResultsThe noninvasive monitoring of pO2 in the tumor tissue after PDT showed that the higher ΔpO2 (pO2 after PDT minus pO2 before PDT), the greater the inhibition of tumor growth. ΔpO2 also correlated with higher levels of the photosensitizers in the tumor and with the occurrence of a severe edema/erythema after PDT.ConclusionMonitoring of PDT-induced changes in tumor oxygenation is a valuable prognostic factor and could be also used to identify potentially resistant tumors, which is important in predicting long-term treatment response.

Photoacoustic Lifetime Imaging of Hypoxia.

We present an approach based on photoacoustic lifetime imaging (PALI) to map the distribution of oxygen partial pressure (pO2) in tissue. This method utilizes methylene blue, a dye widely used in clinical applications, as an oxygen-sensitive imaging agent. PALI measurement of oxygen relies upon pO2-dependent excitation lifetime of the dye. The technique maps the excited triplet state of oxygen-sensitive dye, thus reflecting the spatial and temporal distributions of tissue oxygen.

The role of adipose tissue and subsequent liver tissue hypoxia in obesity and early stage metabolic dysfunction associated steatotic liver disease.

Obesity is linked to several health complication, including Metabolic Dysfunction Associated Steatotic Liver Disease (MASLD). Adipose tissue hypoxia has been suggested as an important player in the pathophysiological mechanism leading to chronic inflammation in obesity, and in the progression of MASLD. The study aims to investigate the effect of progressive obesity on adipose and liver tissue hypoxia. Male 8-week-old C57BL/6J mice were fed a high-fat high-fructose diet (HFHFD) or control diet (CD) for 4, 8, 12, 16 and 20 weeks. Serum ALT, AST and lipid levels were determined, and glucose and insulin tolerance testing was performed. Liver, gonadal and subcutaneous adipose tissue was assessed histologically. In vivo tissue pO2 measurements were performed in gonadal adipose tissue and liver under anesthesia. A PCR array for hypoxia responsive genes was performed in liver and adipose tissue. The main findings in the liver were validated in another diet-induced MASLD mice model, the choline-deficient L-amino acid defined high-fat diet (CDAHFD). HFHFD feeding induced a progressive obesity, dyslipidaemia, insulin resistance and MASLD. In vivo pO2 was decreased in gonadal adipose tissue after 8 weeks of HFHFD compared to CD, and decreased further until 20 weeks. Liver pO2 was only significantly decreased after 16 and 20 weeks of HFHFD. Gene expression and histology confirmed the presence of hypoxia in liver and adipose tissue. Hypoxia could not be confirmed in mice fed a CDAHFD. Diet-induced obesity in mice is associated with hypoxia in liver and adipose tissue. Adipose tissue hypoxia develops early in obesity, while liver hypoxia occurs later in the obesity development but still within the early stages of MASLD. Liver hypoxia could not be directly confirmed in a non-obese liver-only MASLD mice model, indicating that obesity-related processes such as adipose tissue hypoxia are important in the pathophysiology of obesity and MASLD.

Augmentation of tumour perfusion by ultrasound and microbubbles: A preclinical study

BackgroundHypoperfusion and the resulting hypoxia in solid tumours are critical causes of treatment resistance. Ultrasound-stimulated microbubbles (USMB) enhance tumour perfusion in a mechanism named the “sononeoperfusion” effect, which may relieve tumour hypoperfusion and hypoxia. The aim of this study was to determine the optimal mechanical index (MI) and therapeutic ultrasound exposure time for the sononeoperfusion effect and preliminarily explore the mechanism of sononeoperfusion and its effect on tumours. MethodsA total of 155 mice bearing MC38 tumours were included in this study. A modified diagnostic ultrasound and microbubbles (Zhifuxian) was used for USMB treatment. Tumour perfusion was evaluated by contrast-enhanced ultrasound (CEUS) and Hoechst 33342. The therapeutic pulse was operated with MIs of 0.1 to 0.5. The ultrasound exposure time was set from 150 s to 600 s. Endothelial nitric oxide synthase (eNOS) inhibition and NO, ATP, and phospho-eNOS (p-eNOS) detection were performed to explore the mechanisms of sononeoperfusion. Hypoxia-inducible factor-1α (HIF-1α) and tumour oxygen partial pressure (pO2) represent hypoxic tumour conditions. ResultsTumour perfusion was increased after USMB treatment at MIs of 0.1–0.4 and ultrasound exposure times of 150 s to 600 s, with optimal augmentation achieved at an MI of 0.3 and ultrasound exposure time of 450 s. The mean fluorescence intensity of Hoechst 33342 after USMB treatment was stronger than that of the control group. Biochemical assays showed a significant increase in ATP, p-eNOS and NO after USMB treatment. PO2 in tumour tissue increased significantly after USMB treatment and was maintained for more than 20 min. ConclusionsThe best sononeoperfusion effect was obtained with an MI of 0.3 and an ultrasound exposure time of 450 s. The effect is most likely related to NO and ATP increases. The sononeoperfusion effect might be a novel way to ameliorate tumour hypoperfusion and hypoxia.

Serial Measurements of Serum Glial Fibrillary Acidic Protein in Moderate-Severe Traumatic Brain Injury: Potential Utility in Providing Insights into Secondary Insults and Long-Term Outcome.

In patients with traumatic brain injury (TBI), serum biomarkers may have utility in assessing the evolution of secondary brain injury. A panel of nine brain-injury- associated biomarkers was measured in archived serum samples over 10 days post-injury from 100 patients with moderate-severe TBI. Among the biomarkers evaluated, serum glial fibrillary acidic protein (GFAP) had the strongest associations with summary measures of acute pathophysiology, including intracranial pressure (ICP), cerebral perfusion pressure (CPP), and brain tissue pO2 (PbtO2). Group based trajectory (TRAJ) analysis was used to identify three distinct GFAP subgroups. The low TRAJ group (n = 23) had peak levels of 9.4 + 1.2 ng/mL that declined rapidly. The middle TRAJ group (n = 48) had higher peak values (31.5 + 5.0 ng/mL) and a slower decline over time. The high TRAJ group (n = 26) had very high, sustained peak values (59.6 + 12.5 ng/mL) that even rose among some patients over 10 days. Patients in the high TRAJ group had significantly higher mortality rate than patients in low and middle TRAJ groups (26.9% vs. 7.0%, p = 0.028). The frequency of poor neurological outcome (Glasgow Outcome Score Extended [GOS-E] 1-4) was 88.5% in the high TRAJ group, 54.2% in the middle TRAJ group, and 30.4% in the low TRAJ group (p < 0.001). ICP was highest in the high TRAJ group (median 17.6 mm Hg), compared with 14.4 mmHg in the low and 15.9 mm Hg in middle TRAJ groups (p = 0.002). High TRAJ patients spent the longest time with ICP >25 mm Hg, median 23 h, compared with 2 and 6 h in the low and middle TRAJ groups (p = 0.006), and the longest time with ICP >30 mm Hg, median 5 h, compared with 0 and 1 h in the low and middle TRAJ groups, respectively (p = 0.013). High TRAJ group patients more commonly required tier 2 or 3 treatment to control ICP. The high TRAJ group had the longest duration when CPP was <50 mm Hg (p = 0.007), and PbtO2 was <10 mm Hg (p = 0.002). Logistical regression was used to study the relationship between temporal serum GFAP patterns and 6-month GOS-E. Here, the low and middle TRAJ groups were combined to form a low-risk group, and the high TRAJ group was designated the high-risk group. High TRAJ group patients had a greater chance of a poor 6-month GOS-E (p < 0.0001). When adjusting for baseline injury characteristics, GFAP TRAJ group membership remained associated with GOS-E (p = 0.003). When an intensive care unit (ICU) injury burden score, developed to quantify physiological derangements, was added to the model, GFAP TRAJ group membership remained associated with GOS-E (p = 0.014). Mediation analysis suggested that ICU burden scores were in the causal pathway between TRAJ group and 6-month mortality or GOS-E. Our results suggest that GFAP may be useful to monitor serially in moderate-severe TBI patients. Future studies in larger cohorts are needed to confirm these results.

Distribution of Oxygen Tension on Microvessels and in Tissue of Rat Brain Cortex at Severe Arterial Hypocapnia

Arterial hypocapnia (AH), induced by voluntary or forced hyperventilation of the lungs, is accompanied by a decrease in cerebral blood flow (due to an increase in the arteriole tone) and an increase in the affinity of hemoglobin for oxygen. As a result, an insufficient oxygen supply to cortical tissue take place and zones with a critically low oxygen tension (pO2) are formed in brain tissue. The distribution of pO2 to cerebral cortex during AH has not been studied enough. The aim of the work was to evaluate the effectiveness of oxygen supply to brain tissue at the level of arterial and venous microvessels at AH. To do this, the following tasks were set: 1) to study the distribution of the pO2 on the arterial and venous microvessels of the rat cerebral cortex; 2) to analyze tissue pO2 profiles near the walls of these microvessels. On anesthetized Wistar rats under conditions of forced hyperventilation (PaCO2 = 17.1 ± 0.7 mm Hg), the distribution of oxygen tension on the wall of pial and radial arterioles with a lumen diameter of 7–70 μm and on the wall of pial and ascending venules with a lumen diameter of 7–300 µm was studied. In tissue, near the wall of cortical arterioles and venules with a lumen diameter of 10–20 μm, tissue pO2 profiles were measured. Measurements of pO2 during spontaneous breathing of the animal with air served as a control. All pO2 measurements were made using platinum polarographic microelectrodes with a tip diameter of 3–5 μm. Visualization of the electrode tip and microvessels was carried out using a LUMAM-K1 microscope with epiobjectives of the contact type. This work presents for the first-time direct measurements of pO2 on the walls of arterioles and venules of the rat cerebral cortex and in tissues at different distances from the walls of these microvessels at AH. It has been shown that AH results in significant decrease in the oxygen supply to cerebral cortex, that is manifested by a significant drop of the pO2’s on venous microvessels and in tissue in the immediate vicinity of the studied microvessels. It has been shown, that the role of arterioles as a direct source of oxygen to brain tissue, is significantly reduced during arterial hypocapnia. Forced hyperventilation results in significant deterioration of oxygen supply to cerebral cortex, despite elevated pO2 values in the systemic arterial blood and in blood of systemic cerebral veins (sagittal sinus).

Chronic intermittent hypoxia promotes glomerular hyperfiltration and potentiates hypoxia-evoked decreases in renal perfusion and PO2.

Introduction: Sleep apnea (SA) is highly prevalent in patients with chronic kidney disease and may contribute to the development and/or progression of this condition. Previous studies suggest that dysregulation of renal hemodynamics and oxygen flux may play a key role in this process. The present study sought to determine how chronic intermittent hypoxia (CIH) associated with SA affects regulation of renal artery blood flow (RBF), renal microcirculatory perfusion (RP), glomerular filtration rate (GFR), and cortical and medullary tissue PO2 as well as expression of genes that could contribute to renal injury. We hypothesized that normoxic RBF and tissue PO2 would be reduced after CIH, but that GFR would be increased relative to baseline, and that RBF, RP, and tissue PO2 would be decreased to a greater extent in CIH vs. sham during exposure to intermittent asphyxia (IA, FiO2 0.10/FiCO2 0.03). Additionally, we hypothesized that gene programs promoting oxidative stress and fibrosis would be activated by CIH in renal tissue. Methods: All physiological variables were measured at baseline (FiO2 0.21) and during exposure to 10 episodes of IA (excluding GFR). Results: GFR was higher in CIH-conditioned vs. sham (p < 0.05), whereas normoxic RBF and renal tissue PO2 were significantly lower in CIH vs. sham (p < 0.05). Reductions in RBF, RP, and renal tissue PO2 during IA occurred in both groups but to a greater extent in CIH (p < 0.05). Pro-oxidative and pro-fibrotic gene programs were activated in renal tissue from CIH but not sham. Conclusion: CIH adversely affects renal hemodynamic regulation and oxygen flux during both normoxia and IA and results in changes in renal tissue gene expression.

Calculation of Tissue Oxygenation via an Inverse Boundary Problem for Transcutaneous Oxygenation Wearable Applications.

In this article, we present a toolset to fully leverage a previously developed transcutaneous oxygenation monitor (TCOM) wearable technology to accurately measure skin oxygenation values. We describe numerical models and experimental characterization techniques that allow for the extraction of precise tissue oxygenation measurements. The numerical model is based on an inverse boundary problem of the parabolic equation with Dirichlet boundary conditions. To validate this model and characterize the diffusion of oxygen through the oxygen sensing materials, we designed a series of control/calibration experiments modeled after the device's clinical application using oxygenation values in the physiological range expected for healthy tissue. Our results demonstrate that it is possible to obtain accurate tissue pO2 measurements without the need for long equilibration times with a small wearable device.

Type 1 respiratory failure-induced hypoxemia triggers complex signaling responses and apoptosis in the rat brain

Type 1 respiratory failure (T1RF)is caused by lung injury or low partial pressure of O 2 (PO 2 ) in air. It is associated with secondary acute brain injury (sABI), but the mechanisms behind it are not clear and could include primary hypoxic injury, increased intracranial pressure due to hypoxemia or due to mechanical ventilation, or could be mediated by lung injury-triggered inflammation. This study aimed to elucidate the effects of T1RF-induced hypoxemia on the brain in the absence of lung injury. SD rats were exposed to 8%O 2 in N 2 for up to 48h. We measured tissue PO 2 (Pt O2 ) and cerebral blood flow (CBF) in the cerebral cortex (CC) and estimated lactate concentrations and apoptosis in the CC, ependymal layer (EL) and choroid plexuses (CPs) during the time course of hypoxemia using the TUNEL assay. Cerebrospinal fluid (CSF) samples were collected during normoxia, after 24h and 48h hypoxemia. The concentrations of 32 signaling molecules were estimated in these samples by multiplex assays. Expressions of 84 signaling molecules and 5 housekeeping genes in the CC were explored by RT 2 Profiler TM PCR Array. Numerical values are presented as mean (SD). Data was compared with one-way ANOVA and the Kruskal-Wallis test. The Mann-Kendall test was used to assess whether CBF was increasing or decreasing during hypoxemia. Fold changes in the expressions of the tested mRNAs were estimated using GeneGlobe software. Statistical significance was set at p&lt;0.05. Hypoxemia exerted a significant effect on Pt O2 in the CC (p&lt;0.01), which was 49.5 (2.99) mmHg (n=10) during normoxia but declined to 6.4 (3.8) mmHg (n=5) after 1h hypoxemia (p&lt;0.01 vs. normoxia) and then slowly increased, reaching 18.5 (2.8) mmHg (n=6) after 43h hypoxemia (p&lt;0.01 vs. normoxia and 1h hypoxemia). This was accompanied by a significant increase in lactate concentrations in the CC and CSF (p&lt;0.01), but not in plasma (p=0.055). Hypoxemia exerted significant effect on the number of apoptotic cells in the CC (p&lt;0.01), EL (p&lt;0.01) and CPs (p&lt;0.01). In the CC the number of apoptotic cells was 5.63 (4.76) during normoxia, 20.48 (4.1) % after 24h hypoxemia (p&lt;0.01 vs. control) and 32.37 (6.5) % after 48h hypoxemia (p&lt;0.01 vs. control, p&lt;0.05 vs. 24h hypoxemia). Any monotonic increase or decrease in the CBF during hypoxemia could not be detected (p&gt;0.05). There was a significant effect of the duration of hypoxemia on the concentrations for the following cytokines in the CSF: IL-15, IL-17A, CXCL10, VEGF, Erythropoietin, FGF21, FSTL-1, GDF8/Myostatin, LIF and SPARC. When compared to normoxia, the expression of 11 mRNAs, most of them for pro-inflammatory cytokines, were 2-2.5 folds lower after 6h hypoxemia. Expression of mRNAs for angiogenic VEGF and pro-inflamatory IL7 were 2 folds and 2-5 folds higher after 24h and 48h hypoxemia, respectively. Our findings revealed a complex response by thebrain during T1RF-induced hypoxemiaand suggest an association between TIRF-induced hypoxemia, tissue hypoxia and sABI. This study was supported by the Kuwait University Research Sector Grants no YM 05/18 and MY 02/18. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

More of the same? Voxelotor spawns a successor, but on what success does it build?

More of the same? Voxelotor spawns a successor, but on what success does it build?

Modeling the impact of tissue oxygen profiles and oxygen depletion parameter uncertainties on biological response and therapeutic benefit of FLASH.

Ultra-high dose rate (FLASH) radiation has been reported to efficiently suppress tumor growth while sparing normal tissue; however, the mechanism of the differential tissue sparing effect is still not known. Oxygen has long been known to profoundly impact radiobiological responses, and radiolytic oxygen depletion has been considered to be a possible cause or contributor to the FLASH phenomenon. This work investigates the impact of tissue pO2 profiles, oxygen depletion per unit dose (g), and the oxygen concentration yielding half-maximum radiosensitization (the average of its maximum value and one) (k) in tumor and normal tissue. We developed a model that considers the dependent relationship between oxygen depletion and change of radiosensitivity by FLASH irradiation. The model assumed that FLASH irradiation depletes intracellular oxygen more rapidly than it diffuses into the cell from the extracellular environment. Cell survival was calculated based on the linear quadratic-linear model and the radiosensitivity related parameters were adjusted in 1Gy increments of the administered dose. The model reproduced published experimental data that were obtained with different cell lines and oxygen concentrations, and was used to analyze the impact of parameter uncertainties on the radiobiological responses. This study expands the oxygen depletion analysis of FLASH to normal human tissue and tumor based on clinically determined aggregate and individual patient pO2 profiles. The results show that the pO2 profile is the most essential factor that affects biological response and analyses based on the median pO2 rather than the full pO2 profile can be unreliable and misleading. Additionally, the presence of a small fraction of cells on the threshold of radiobiologic hypoxia substantially alters biological response due to FLASH oxygen depletion. We found that an increment in the k value is generally more protective of tumor than normal tissue due to a higher frequency of lower pO2 values in tumors. Variation in the g value affects the dose at which oxygen depletion impacts response, but does not alter the dose-dependent response trends, if the g value is identical in both tumor and normal tissue. The therapeutic efficacy of FLASH oxygen depletion is likely patient and tissue-dependent. For breast cancer, FLASH is beneficial in a minority of cases; however, in a subset of well oxygenated tumors, a therapeutic gain may be realized due to induced normal tissue hypoxia.

The effects of recruitment of renal functional reserve on renal cortical and medullary oxygenation in non-anesthetized sheep.

Recruitment of renal functional reserve (RFR) with amino acid loading increases renal blood flow and glomerular filtration rate. However, its effects on renal cortical and medullary oxygenation have not been determined. Accordingly, we tested the effects of recruitment of RFR on renal cortical and medullary oxygenation in non-anesthetized sheep. Under general anesthesia, we instrumented 10 sheep to enable subsequent continuous measurements of systemic and renal hemodynamics, renal oxygen delivery and consumption, and cortical and medullary tissue oxygen tension (PO2 ). We then measured the effects of recruitment of RFR with an intravenous infusion of 500 ml of a clinically used amino acid solution (10% Synthamin® 17) in the non-anesthetized state. Compared with baseline, Synthamin® 17 infusion significantly increased renal oxygen delivery mean ± SD maximum increase: (from 0.79 ± 0.17 to 1.06 ± 0.16 ml/kg/min, p < 0.001), renal oxygen consumption (from 0.08 ± 0.01 to 0.15 ± 0.02 ml/kg/min, p < 0.001), and glomerular filtration rate (+45.2 ± 2.7%, p < 0.001). Renal cortical tissue PO2 increased by a maximum of 26.4 ± 1.1% (p=0.001) and medullary tissue PO2 increased by a maximum of 23.9 ± 2.8% (p=0. 001). In non-anesthetized healthy sheep, recruitment of RFR improved renal cortical and medullary oxygenation. These observations might have implications for the use of recruitment of RFR for diagnostic and therapeutic purposes.

GAT inhibition preserves cerebral blood flow and reduces oxidant damage to mitochondria in rodents exposed to extreme hyperbaric oxygen.

Oxygen breathing at elevated partial pressures (PO2's) at or more than 3 atmospheres absolute (ATA) causes a reduction in brain γ-aminobutyric acid (GABA) levels that impacts the development of central nervous system oxygen toxicity (CNS-OT). Drugs that increase brain GABA content delay the onset of CNS-OT, but it is unknown if oxidant damage is lessened because brain tissue PO2 remains elevated during hyperbaric oxygen (HBO2) exposures. Experiments were performed in rats and mice to measure brain GABA levels with or without GABA transporter inhibitors (GATs) and its influence on cerebral blood flow, oxidant damage, and aspects of mitochondrial quality control signaling (mitophagy and biogenesis). In rats pretreated with tiagabine (GAT1 inhibitor), the tachycardia, secondary rise in mean arterial blood pressure, and cerebral hyperemia were prevented during HBO2 at 5 and 6 ATA. Tiagabine and the nonselective GAT inhibitor nipecotic acid similarly extended HBO2 seizure latencies. In mice pretreated with tiagabine and exposed to HBO2 at 5 ATA, nuclear and mitochondrial DNA oxidation and astrocytosis was attenuated in the cerebellum and hippocampus. Less oxidant injury in these regions was accompanied by reduced conjugated microtubule-associated protein 1A/1B-light chain 3 (LC3-II), an index of mitophagy, and phosphorylated cAMP response element binding protein (pCREB), an initiator of mitochondrial biogenesis. We conclude that GABA prevents cerebral hyperemia and delays neuroexcitation under extreme HBO2, limiting oxidant damage in the cerebellum and hippocampus, and likely lowering mitophagy flux and initiation of pCREB-initiated mitochondrial biogenesis.

Brain Tissue Oxygen and BOLD fMRI Under Different Levels of Neuronal Activity.

Localized increases in neuronal activity are supported by the hemodynamic response, which delivers oxygen to the brain tissue to support synaptic functions, action potentials and other neuronal processes. However, it remains unknown if changes in baseline neuronal activity, which are expected to reflect neuronal metabolic demand, alter the relationship between the local hemodynamic and oxygen behaviour. In order to better characterize this system, we examine here the relationship between brain tissue oxygen (PO2) and hemodynamic responses (BOLD functional MRI) under different levels of neuronal activity. By comparing the stimulus-evoked responses during different levels of baseline neuronal activity, the awake state vs isoflurane anesthesia, we were able to measure how a known change in neuronal demand affected tissue PO2 as well as the hemodynamic response to stimulation. We observed a high correlation between stimulus-evoked PO2 and BOLD responses in the awake state. Moreover, we found that the evoked PO2 and BOLD responses were still present despite the elevated tissue oxygen baseline and decreased baseline of neuronal activity under low concentration isoflurane,and that the magnitudes of these responses decreased by similar proportions but the relationship between these signals was distorted. Our findings point to distortion of the BOLD-PO2 relationship due to anesthesia. The feedback mechanism to adjust the level of brain tissue oxygen, as well as the correlation between BOLD and PO2 responses, are impaired even by a small dose of anesthetics.